1. Field of the Invention
The present invention relates to an oxide thin film for a bolometer, particularly to an oxide thin film for a bolometer making it possible to make the temperature coefficient of its electric resistance large, a process for producing the same, and a non-cooling type infrared sensor using the same.
2. Description of the Related Art
Hitherto, there has been known a bolometer style, non-cooling type infrared sensor, which is a sensor of absorbing incident infrared rays in its light-receiving portion to cause a change in temperature, and then detecting the radiation intensity of the incident infrared rays by sensing a change in electric resistance generated by the temperature change. A bolometer is a member using a temperature change in the electric resistance of a metal or semiconductor thin film thermally insulated from a substrate material. Properties required for the material for this bolometer are, for example, electric resistance and the temperature coefficient of the electric resistance (referred to as TCR hereinafter). As the electric resistance of the bolometer material becomes larger, Johnson noises in general become larger. Such a case is unfavorable. On the other hand, when the electric resistance becomes small, a difference between the wiring resistance of other than the bolometer and the electric resistance of the bolometer material becomes small. Such a case is also unfavorable. Therefore, it is desirable that the electric resistance of the bolometer material is from about 5 to about 100 kxcexa9 at room temperature. In other words, when the thickness of the resistor thin film for the bolometer is set to 50 to 1000 nm, the electric resistivity required for the bolometer material is desirably from about 0.025 to about 10 xcexa9cm.
The temperature resolution power (referred to as NETD hereinafter) of an infrared sensor is in inverse proportion to the absolute value of the TCR of the bolometer material thereof. Therefore, by using a bolometer material whose absolute value of TCR is large, an infrared sensor having a small NETD can be obtained. Since the TCR of alloy thin films made of nickel iron alloy or the like is generally as small as about 0.5%/K, the films are not preferred as bolometer materials for high-sensitivity infrared sensors. On the other hand, as disclosed in Japanese unexamined patent publication (KOKAI) No.11-271145, the TCR of a vanadium oxide thin film is relatively large, that is, about 2%/K; therefore, the vanadium oxide thin film is used as a bolometer material. As disclosed in Japanese unexamined patent publication (KOKAI) No.2000-143243, a trial is made wherein a part of vanadium V is replaced by some other element, for example, manganese Mn. This publication reports that the absolute value of the TCR thereof can be improved up to about 4%/K.
As disclosed in, for example, Japanese unexamined patent publication (KOKAI) No.2000-95522, it is investigated to use a unique property that a perovskite Mn oxide such as La1xe2x88x92xSrxMnO3 undergoes phase-transition from a high-resistance semiconductor state at high temperature to a low-resistance metal state at low temperature with a change in magnetic property of the oxide. The temperature at which this phase-transition arises can be set to room temperature or similar temperature by adjusting, for example, the composition x of Sr. Since a large change in the electric resistance thereof is caused with this phase-transition, a large TCR can be obtained. In fact, it is reported that this material can give a high TCR of 5%/K or more, particularly about 10%/K. Thus, it is expected that this material is applied to infrared sensors.
As described above, a vanadium oxide thin film can give a TCR of about 2 to 4%/K by an improvement such as a replacement of the element therein. If the phase-transition of perovskite Mn oxides is used, a relatively large TCR of about 5 to 10%/K can be obtained. Therefore, in conventional non-cooling type infrared sensors, it has been investigated to use these materials for resistor thin films for a bolometer. However, in order to make infrared sensors more sensitive and increase the number of pixels thereof from now, it is necessary to develop bolometer materials having a large TCR.
In light of this situation, an object of the present invention is to provide an oxide thin film for a bolometer making it possible to make the temperature coefficient of its electric resistance large, a process for producing the same, and an infrared sensor.
The oxide thin film for a bolometer of the present invention comprises a cobalt-based oxide represented by YBaCo2O5.5+x (xe2x88x920.5 less than x less than 0.05).
In one example of the oxide thin film for the bolometer of the present invention, at least one part of the Y element in the cobalt-based oxide is replaced by at least one element selected from the group of Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho, or at least one compound comprising at least one element selected from the same group.
In one example of the oxide thin film for the bolometer of the present invention, at least one part of the element Ba in the cobalt-based oxide is replaced by at least one selected from the group of Sr and Ca, or at least one compound comprising at least one element selected from the same group.
In one example of the oxide thin film for the bolometer of the present invention, the cobalt-based oxide is formed on an insulating substrate, and this insulating substrate is composed of a thin layer of a perovskite oxide monocrystal. The perovskite oxide monocrystal thin layer is made of SrTiO3, LaAlO3, NdGaO3, or the like.
The process for producing an oxide thin film for a bolometer of the present invention is a process wherein a sol-gel process is used to form the above-mentioned oxide thin film for the bolometer on an insulating substrate.
The process for producing an oxide thin film for a bolometer of the present invention is a process wherein a physical film-forming process is used to form the above-mentioned oxide thin film for the bolometer on an insulating substrate. Examples of the physical film-forming process include sputtering method, and laser ablation method.
The process for producing an oxide thin film for a bolometer of the present invention is a process wherein a solution in which an organic metal compound is dissolved in a solvent, this solvent is applied onto an insulating substrate, the applied solution is dried and subsequently a laser ray is radiated onto the solution to crystallize the solution, thereby forming the above-mentioned oxide thin film for the bolometer.
The infrared sensor of the present invention is a sensor wherein the above-mentioned oxide thin film for the bolometer is used as a resistor (4) for the bolometer.
The infrared sensor of the present invention is a sensor which has a microbridge structure wherein the above-mentioned oxide thin film for the bolometer is thermally separated from a semiconductor substrate (1).